1. Field of the Invention
This invention is in the field of devices for injecting precisely metered doses of pharmaceutical agents. The invention particularly relates to a hypodermic syringe having the same general appearance as a pen or mechanical pencil which is specifically adapted to provide for multiple measured doses of agents such as insulin or human growth hormone.
2. Description of the Prior Art
Diabetics and others frequently find themselves in situations where the assistance of a health professional is unavailable to administer a subcutaneous or intramuscular injection of measured amount of a liquid agent. In such situations such persons need to have a low-cost syringe which does not require the assistance of a health professional to achieve the desired measure of accuracy. It is often the case that such persons require more than one dose per day, each dose being of a somewhat different volume. Dispensers of this general type are known which are the general appearance of a pen or mechanical pencil. The dispenser is typically large enough to hold several such doses, yet it is small enough to fit conveniently in one's pocket or purse. Examples of such devices are to be found in U.S. Pat. Nos. 4,973,318; 4,413,760; 4,498,904; 3,613,952; 4,475,905; 4,883,472; and 4,592,745. Additional examples are show in PCT International Publications WO 87/02895; WO 88/07874; WO 82/02662; PCT CH 86/00151; PCT DK 88/0064.
In devices of this class, a container of the liquid is generally provided having a closed first end adapted to be penetrated by a needle assembly so as to permit the liquid in the container to pass out the injection. The second end of the container is generally closed by a piston. To prevent tampering or reuse of the liquid container, the piston is generally designed such that a pushing force can be applied to the piston to reduce the liquid-holding volume of the container, but no feature is presented which would be suitable for pulling on the piston so as to enlarge the liquid-holding volume of the container.
An elongated member in the nature of a plunger rod is received within the housing for exerting a force on the piston closing the second end of the container. A means is provided for measuring the distance which the plunger rod travels to determine the decrease in volume of the liquid container which causes the dispensing of the liquid within the container. It has generally been recognized that the dispenser should have some feature which would allow the rod to only travel in a single direction toward the piston thereby preventing any action on the part of the rod which might permit an enlargement Of the volume of the liquid container. A safety cover is generally provided over a needle assembly attached to the closed end of the container.
While the prior art pen-style syringes have met with some success, certain shortcomings have also been observed. In some prior art pens it is difficult to be sure that after a dose has been administered there is not a back-up which unintentionally alters the length of the piston stroke for the next dose.